The present invention relates generally to the measurement of a specific ion activity or concentration in solution by potentiometric measuring, recording and controlling systems in which the ion concentration is sensed by an electrode measuring assembly or reference cell which is electrically connected to a suitable measuring meter, and more particularly to the method for making and the embodiment of a deactivated pH or other ion sensitive glass electrode which when combined or paired as a half-cell with a normal pH or other ion sensitive glass electrode will provide an electrode measuring assembly or reference cell having the characteristics of a theoretically perfect reference electrode cell and therefore is adapted for use in such potentiometric measuring, recording and controlling systems for measuring pH or other ion concentrations in a given sample solution.
It is well known in the prior art that hydrogen ion concentration in a given sample solution can be measured with a pH measuring system. In todays conventional pH measuring systems used in industrial processes such as water conditioning and waste control, these systems include, an electrode measuring assembly or reference electrode cell to sense the pH which consists of a normal pH sensitive glass electrode half-cell coupled with a reference electrode half-cell of the flowing or non-flowing junction type and sometimes such units include a temperature sensor. The electrode measuring assembly is electrically connected either directly or through an amplifying arrangement to a pH meter having a suitable electromotive force thereacross and the pH meter reads out the measurements in millivolts. The readings which represent hydrogen ion activity in the solution being measured can be extrapolated to provide the hydrogen ion concentration.
These systems and the parts therefor are well known to those skilled in the art and the systems and their parts are available in the commercial marketplace from various manufacturers such as Beckman Industrial Corporation of Fullerton, Calif. and Cedar Grove, N.J. Therefor when the description set forth herein including the examples refer to a normal pH sensitive glass electrode, it is intended to mean electrodes of the type which are sold in the commercial marketplace by Beckman Industrial Corporation.
The normal pH sensitive glass electrode used in these systems is a generally elongated hollow, cylindrical glass member having the shape of a test tube which is closed at the immersion end and open at the opposite or remote end. The closed or immersion end of the pH glass electrode has a dome, bulb or other suitable configuration and extending through a suitable sealing means for the open end of the glass member and downwardly along the medial portion of the hollow inner chamber formed in the elongated glass member of the pH sensitive glass electrode is a suitable conductive wire element surrounded in assembled position by a buffered filling solution having a generally constant hydrogen ion and chloride ion concentration which is sealed in the inner chamber.
The glass wall of the elongated glass members of the pH sensitive glass electrode must meet many requirements. For example, it must during operation provide a span of nearly ideal millivolt readings for each given change in pH unit throughout the entire pH scale with little error in very acid or very alkaline solutions. Generally the composition of the glass membrane or wall of a pH sensitive glass electrode will consist of some combination of univalent, bivalent or trivalent metal or metals, and silicon dioxide mixed in varying ratios by weight to produce the desired compromise between response and resistance which is inherent for the thickness of each particular composition of glass used for the glass wall of the elongated glass member. In practice, manufacturers will produce several types of pH sensitive glass electrodes some with a narrow range, for example a pH range from 0 to 11 units; and others with a full pH range from 0 to 14 units.
In theory, it is believed that the ability of a particular pH sensitive glass electrode to respond to change in the hydrogen ion activity in a given solution is a function of the water content of the composition selected for the wall of the given glass member. The surface or more particularly the insertion tip end of the pH sensitive glass electrode swells slightly when immersed in a solution to be tested. In theory again it is beleived that a hydrated layer is formed as the water solution penetrates into the silicate network of the glass membrane or wall portion of the pH sensitive glass electrode immersed in the solution being tested and that this hydrated layer tends to facilitate migration of ions because it lowers the electrical resistivity of the glass wall; to the extent that an ion exchange equilibrium can be established across the phase-boundry between the hydrated layer and the glass wall; by the hydrogen ions in the solution being tested and the alkali metal ions in the glass wall of the normal pH sensitive glass electrode. This equilibrium is the potential established in the pH glass sensitive electrode which is measured and this potential varies in a known manner with the change in hydrogen ion activity, as distinguished from the concentration of the hydrogen ion in the solution being measured.
The reference electrode supplies a stable reference potential against which the measured potential developed across the glass wall of the pH sensitive glass electrode may be compared and effectively used for analytical purposes in various ways.
The reference electrodes constructed in accordance with known prior art concepts of fabrication and design suffer from significant limitations. Thus prior art reference electrodes which incorporate a flowing junction while exhibiting low junction potential must be continually replenished with internal electrolyte and unless counter pressurized are subject to intrusion from the solution or process fluid being measured. Non-flowing reference electrodes exhibit high junction potentials and are prone to junction plugging by deposits collected on the outer surface from the solution or process fluid being measured. Last, the electrolytic bridge present in both flowing and non-flowing reference electrodes is a necessary, but undesirable feature, since it enables cross diffusion of the solution or process fluid being measured and the internal electrolyte of the reference electrode which results in contamination of both the solution or process fluid and contamination of the internal electrolyte in the reference electrode half cell compartment.
The combination of the pH sensitive glass electrode half cell with the reference electrode half cell provides an electrode measuring assembly or cell to complete the measuring circuit read by the pH meter which is essentially a sensitive potentiometer acting in the circuit to balance off the differences in the respective potential in the pH sensitive glass electrode and the reference electrode. The pH meter reading will be either negative or positive depending upon the hydrogen ion activity in the solution being measured. It will provide a millivolt reading from which the equivalent hydrogen ion concentration for the solution can be extrapolated.
It has been found that when a normal unfilled or filled pH sensitive electrode is exposed to high temperature for a predetermined period of time; in either a dry temperature controlled environment such as an oven or in a wet temperature controlled environment such as in an immersion liquid in a pressurized autoclave, the nominal electrode senstivity of the normal pH sensitive electrode is reduced to very low levels and that the output potentials of such deactivated pH glass electrodes approach the characteristics desired for a theoretically perfect reference electrode.
Because of these unusual characteristics, a deactivated glass electrode can be combined or coupled with a conventional pH or ion sensitive glass electrode to provide an electrode measuring assembly or cell which when calibrated in suitable buffer solutions will be adapted for use in a potentiometric measuring system in place of the conventional known prior art electrode measuring assemblies with flowing or non flowing junction type reference electrode half cells such that
(a) the respective glass membranes or walls prevent fluid transport between the electrodes for the respective half cells of the combination electrode assembly, PA1 (b) eliminate the requirement for refilling such as is needed for a flowing junction type of reference electrode, PA1 (c) prevents cross contamination because the electrodes in the respective half cells are separated by their glass membranes or walls, PA1 (d) insures that the respective temperature coefficients are at all times equal in the respective electrodes by the symmetry of the respective pH sensitive glass electrode and the deactivated pH glass electrode and the close proximity of the electrodes in the sample solution being measured,
Further, it has been determined that deactivation or degradation of a pH or ion sensitive glass electrode is a function of the bulk property of the glass, not a surface phenomenon to the extent that erosion of the outer surface of the glass membrane or wall in the dome or bulb of the immersion end of the pH sensitive glass electrode due to etching or abrasion will not change the characteristics of the deactivated glass electrode insofar as its pH or ion response sensitivity is concerned.
Since the deactivation or degradation of a pH sensitive glass electrode is a function of the bulk property of the glass and not a surface phenomenon, it has been concluded that exposing the glass walls of the filled or unfilled pH sensitive glass electrode causes sufficient dehydration of the glass wall to deplete alkaline ions and the like pH sensitive sites on the outer surface and in the wall of the glass electrode. As a result, the normal pH sensitive glass electrode becomes a high-impedence structure which is difficult to rehydrate because of the missing alkaline ions. Such a deactivated pH or other ion sensitive glass electrode will be insensitive to changes in the pH or to other ion concentration in the solution being measured or monitored.
As another possibility, it has been hypothesized that when the conventional pH sensitive glass electrode is exposed to intense thermal conditions for a given period of time that the glass electrode is converted or forms hydrated silica sites or a silicon/dioxide/water mixture on the outer surface and in the wall defining the glass electrode.